A silicon wafer, which is a material of a semiconductor device, is cut out from a silicon single crystal, lapped, and then polished so as to have a mirror surface. This mirror finish was provided only on a device formation surface, but for wafers of a large diameter exceeding 8 inches, for example, 12-inch wafers, there has been a need to finish them in such a manner that their rear surface, on which no device is formed, is comparable to a mirror one. It has correspondingly been necessary to polish both surfaces of the wafers.
A planetary gear-based double side polishing apparatus is normally used for double side polishing of a silicon wafer. The structure of this double side polishing apparatus will be described in brief with reference to FIGS. 26 and 27. FIG. 27 is taken along a line C-C in FIG. 26 which is indicated by arrows.
The planetary gear-based double side polishing apparatus comprises a vertical pair of rotary surface plates 1 and 2, a plurality of carriers 3, 3, . . . arranged around a rotation center between the rotary surface plates 1 and 2 as planetary gears, a sun gear 4 arranged at the rotation center between the rotary surface plates 1 and 2, and an annular internal gear 5 arranged in an outer periphery between the rotary surface plates 1 and 2.
The upper rotary surface plate 1 can be elevated and lowered and rotates in a direction opposite to that for the lower rotary surface plate 2. The rotary surface plates 1 and 2 each have a polishing cloth (not shown) installed on its surface opposite to the other. Each carrier 3 has an eccentric circular accommodation hole in which a circular work 6 comprising a silicon wafer is held. The sun gear 4 and the internal gear 5 engage with the plurality of carriers 3 from the inside and outside, respectively, and are normally driven rotationally in the same direction as the lower rotary surface plate 2.
During a polishing operation, with the upper rotary surface plate 1 lifted, the plurality of carriers 3, 3, . . . are set on the lower rotary surface plate 2 and the work 6 is conveyed into each of the carriers 3, which are then supplied onto the rotary surface plate 2. Once all the works 6, 6, . . . have been provided, the upper rotary surface plate 1 is lowered to sandwich the works 6, 6, . . . between the rotary surface plates 1 and 2, more specifically, between the upper and lower polishing cloths. Then, a grinding liquid is poured between the rotary surface plates 1 and 2 while the sun gear 4 and the internal gear 5 are rotationally driven.
This rotational driving causes the plurality of carriers 3, 3, . . . to rotate between the rotary surface plates 1 and 2, which rotate in opposite directions, while revolving them around the sun gear 4. This allows the plurality of works 6, 6, . . . to be simultaneously polished on both sides.
It is an important technical object to automate such a double side polishing operation for silicon wafers, but the automation has been hindered for the following reasons.
(First Reason)
To automate the double side polishing operation for silicon wafers, for example, the plurality of works 6, 6, . . . must automatically be supplied onto the lower rotary surface plate 2. For this automatic supply, it has been contemplated that with the lower rotary surface plate 2 fixed, a sucking type transfer and loading robot simultaneously or sequentially conveys the works 6, 6, . . . into the plurality of carriers 3, 3, . . . set on the lower rotary surface plate 2.
If, however, the works 6 are 12-inch silicon wafers, the sizes of the rotary surface plates 1 and 2, the internal gear 5, and other peripheral components increase consistently with the size of the work 6 to increase tolerances, resulting in inaccurate positions of the carriers 3, 3, . . . placed on the lower rotary surface plate 2. On the other hand, the tolerance between the inner diameter of the carrier 3 and the outer diameter of the work 6 is more strictly limited. Thus, with the method of mechanically conveying the works 6, 6, . . . into the carriers 3, 3, . . . on the rotary surface plate 2, the work 6 may not completely be fitted in the carrier 3, thereby requiring monitoring and corrections by an operator. This has thus been found to be a major factor for hindering perfect automation.
(Second Reason)
To automate the double side polishing operation for silicon wafers, the plurality of works 6, 6, . . . must not only supplied onto the lower rotary surface 2 but the plurality of polished works 6, 6, . . . must also be automatically ejected from the lower rotary surface plate 2. The automatic ejection is achieved by a sucking type transfer and loading robot by sequentially unloading the works 6, 6, . . . from the carriers 3, 3, . . . on the lower rotary surface plate 2.
For the double side polishing, however, the polished works 6, 6, . . . are in relatively tight contact with the upper and lower polishing clothes. Thus, when the upper rotary surface plate 1 is lifted after the polishing, some of the works 6, 6, . . . may be held on the upper rotary surface plate 1 and may separate from the works 6, 6, . . . remaining on the lower rotary surface plate 2. Of course, such a work separation phenomenon seriously hinders automatic ejection of the works from the lower rotary surface plate 2.
As measures for preventing this work separation phenomenon, it has been contemplated that a plurality of rammers are provided on the upper rotary surface plate 1 in such a fashion as to correspond to the plurality of works 6, 6, . . . between the rotary surface plates 1 and 2 and that when the rotary surface plate 1 is lifted after the polishing, the plurality of rammers mechanically push the plurality of works 6, 6, . . . downward. As additional measures, Japanese Patent Laid-Open No. 9-88920 discloses a technique with which a plurality of suction nozzles are provided on the upper rotary surface plate 1 in such a fashion as to correspond to the plurality of works 6, 6, . . . so that when the rotary surface plate 1 is lifted after the polishing, all the works 6, 6, . . . between the rotary surface plate 1 and 2 are sucked and held on the upper rotary surface plate 1.
Both measures can concentrate all the works 6, 6, . . . on one of the rotary surface plates 1 and 2. The former case, however, may mechanically damage the works 6, 6, . . . after the polishing, and this damage may create a serious problem. Examinations by the inventors show that the latter case does not mechanically stress the works 6, 6, . . . after the polishing but may cause the bottom surfaces of the works 6, 6, . . . separated from the lower rotary surface plate 2 to dry as the upper rotary surface plate 1 rises. This drying is a serious problem with silicon wafers.
(Third Reason)
In such a double side polishing operation for silicon wafers, the polishing clothes installed on the opposite surfaces of the rotary surface plates 1 and 2 are cleaned by means of brushing before the polishing operation. The brushing process is carried out by rotating and revolving brushes shaped like gears with the same outside shape as that of the carriers 3, but the supply and ejection of the brushes is carried out by the operator by manually supplying the brushes onto the lower rotary surface plate 2 and after the operation, ejecting the brushes therefrom.
Since the brushing is not frequently carried out, such manual supply and ejection of the brushes poses no particular problem. Since, however, high polishing quality is required to polish both surfaces of 12-inch silicon wafers, the brushing is required for each polishing operation. It has thus been found that if the brushes are manually supplied and ejected, a resulting decrease in working efficiency and a resulting increase in working costs create a serious problem.
That is, it is an important technical problem to automate the double side polishing of silicon wafers. For this automation, for example, the plurality of works 6, 6, . . . must be automatically supplied onto the lower rotary surface plate 2 and the polished works 6, 6, . . . must be automatically ejected from the lower rotary surface plate 2. The examinations by the inventors, however, have also shown that the manual supply and ejection of the brushes, like the manual supply and ejection of works, may significantly reduce working efficiency and increase working costs and that no effective automated apparatus has been established.
In addition to the brushing, dressing is used as mechanical processing for the polishing cloths. This processing is conventionally carried out to level the surfaces after the polishing cloths have been changed. However, it has been shown that the double side polishing of, for example, 12-inch silicon wafers, which requires a high quality operation, requires one dressing process to be executed at least every several polishing process in order to obtain sufficient quality and that this dressing process also significantly obstruct the automation for double side polishing apparatuses that pursue high quality.
It is an object of the present invention to eliminate the various factors that hinders the automation of the double side polishing operation to enable perfect automation.
That is, it is a first object of the present invention to provide a double side polishing method and apparatus that enable even large-diameter works such as 12-inch silicon wafers to be perfectly automatically supplied onto the lower rotary surface plate.
It is a second object of the present invention to provide a double side polishing method and apparatus that enables works to be automatically ejected from between the upper and lower rotary surface plates while reliably preventing the works from being mechanically damaged or dried.
It is a third object of the present invention to provide a double side polishing apparatus that can efficiently and economically carry out high-quality double side polishing with frequent brushing or dressing.
It is another object of the present invention to provide a double side polishing apparatus that can polish large works accurately, efficiently, and inexpensively while preventing them from being contaminated.
It is still another object of the present invention to provide a double side polishing apparatus that can increase the usage of a grinding liquid supplied between the upper and lower surface plates to preclude it from entering a drive section.
It is yet another object of the present invention to provide a double side polishing apparatus and carriers for use therein that can effectively prevent wafers held in the carriers from being damaged due to idle running.
It is further another object of the present invention to provide a double side polishing apparatus that can prevent contaminations and damages as large as possible, which become problems at the time of forming a device.